Altered hemispheric lateralization of speech-related cortical activity in unilateral severe hearing loss: an fMRI study.

BACKGROUND: Severe unilateral hearing loss is often managed conservatively after initial treatment because hearing in the contralateral ear is preserved. However, patients frequently experience difficulties in speech perception, particularly in noisy environments, that are not fully captured by conventional audiological assessments. Because central auditory pathways project predominantly to the contralateral hemisphere and speech processing is typically left-lateralized, input from the right ear may reach the left-hemispheric speech areas more directly. This anatomical and functional asymmetry suggests that the side of hearing loss could differentially affect speech-related cortical processing. OBJECTIVE: This study aimed to investigate cortical activity during speech perception in severe postlingual unilateral hearing loss and examine the effects of ear-specific input on hemispheric lateralization. METHODS: Functional magnetic resonance imaging (fMRI) was performed in individuals with severe unilateral hearing loss (right- and left-sided) and in normal-hearing controls during a speech perception task. Auditory stimuli (monosyllables and pure tones) were presented binaurally. Region-of-interest analyses were conducted for the primary auditory cortex (transverse temporal gyrus, TTG) and the superior temporal gyrus (STG). The mean activation and laterality indices were compared between the groups. RESULTS: No significant intergroup differences were observed in the primary auditory cortex. In contrast, STG activity was significantly reduced in patients with unilateral hearing loss compared to controls, particularly in the right-sided hearing loss group. The hemispherical lateralization of STG activity was altered, with increased leftward dominance observed in the left-sided hearing loss group. CONCLUSION: Unilateral hearing loss affects higher-order speech-related cortical processing rather than primary auditory responses. The observed changes in STG activity and lateralization suggest that ear-specific auditory input influences the cortical organization. These findings are consistent with the concept of a right-ear advantage and provide neurophysiological insights into speech perception difficulties in unilateral hearing loss.